1. Field of the Invention
The present invention relates to a mask inspecting method for a phase-shifting mask and a mask detector used for the inspection thereof in an exposure step of a process of manufacturing a semiconductor device.
2. Description of the Background Art
A lithography technique is usually used for forming a circuit pattern on a semiconductor substrate such as a wafer in a process of manufacturing a semiconductor device.
A method of manufacturing a semiconductor device using the lithography technique is as follows. A mask pattern of a photomask is transferred to a photoresist coating the surface of a wafer to form a resist pattern. The photoresist is developed, being partially removed according to the pattern, then a portion of the surface with the photoresist off is etched, to form a circuit pattern. Therefore, the accuracy of a mask pattern of the photomask exerts great influence on accuracy and electric characteristics of a semiconductor device.
For this reason, a photomask used in an exposure step of the process of manufacturing a semiconductor device has been conventionally made a sufficient inspection for its defects and drawing accuracy in advance.
FIG. 8 illustrates a principal part of a conventional photomask 70 used in the exposure step of the process of manufacturing a semiconductor device.
In the figure, numeral 71 denotes a glass substrate, on which a light shielding film 72 made of chromium and the like is formed by sputtering or the like.
The light shielding film 72 is provided with a pattern 73 of the similar shape, for example rectangular, to the pattern to be transferred to a photoresist surface on a wafer. (In FIG. 8, only one pattern 73 is illustrated for explanatory convenience.) The pattern 73 is transferred by light having a specific wavelength (ultraviolet region) onto the photoresist formed on a wafer surface to be developed, then through an etching step, an unnecessary photoresist is removed to form a circuit pattern on the wafer.
FIG. 9 is a block diagram of a conventional mask inspecting system 80 for inspecting defects of a pattern of the photomask 70.
A circuit pattern designed by a circuit designer with CAD and the like is stored as pattern data in a pattern data storage 81 made of magnetic disk or the like.
The pattern data are converted into electron beam data (referred to as "EB data" hereinafter) for operating an electron beam lithography system in an EB data generating part 82 to be stored in an EB data storage 83.
The above EB data serve to provide an on/off signal and position data of the electron beam employed in the device for operating an electron beam lithography system.
In a mask manufacturing part 84, the electron beam lithography system operates as a function of the EB data, performing the step of exposure of a resist of a masking material, then followed by the steps of development, etching and so on, thus manufacturing the photomask 70.
The photomask 70 thus manufactured is imaged by an image pickup device using an ordinary light source in a mask image pickup part 85, whereby image data associated with the pattern is obtained.
The mask image pickup part 85 has an image pickup device 90 of light transmissive type using a white light source 91, for example, as shown in FIG. 10.
White light L1 emitted from the light source 91 goes through a condenser lens 92, being converted into a parallel ray L2, to perpendicularly enter a mask surface of the photomask 70.
Light L3 passing through the pattern of the photomask 70 forms an image through a projection lens 93 on a two-dimensional photo array sensor 94, thus detecting a shape of the pattern.
Detection signals from the photo array sensor 94 are each converted into "0" or "1" according to a constant threshold value by a binary circuit 95, to thereby generate binary image data.
Furthermore, the image pickup device used in the mask image pickup part 85 is not limited to the light transmissive type which forms an image of the light passing through the pattern as described above, but it may be a reflective type which projects light onto the photomask 70 to form an image of a reflective light therefrom.
The image data of the pattern of the photomask 70 thus obtained is stored in an image data storage 86.
In a data checking part 87, the EB data and the corresponding image data are read out from the EB data storage 83 and the image data storage 86, respectively, and checked with each other.
The image data, being binary as mentioned above, are formed in the manner that if the value of the image data corresponding to the pattern 73 is "1", for example, that of the light-shielding film 72 is set for "0", so that the image data correspond in composition to the EB data which consist of an on/off signal of electron beam and position data. Therefore, when both data are simultaneously scanning in the same direction by means of CPU and the like, it can be checked whether or not both data coincide with each other by comparing the values of "0" and "1" one by one.
Thus, it can be inspected whether or not the mask pattern of the photomask 711 is precisely formed as a function of the predetermined EB data, i.e., it has any defect in size or shape.
Passing the inspection, the photomask 70 can be used in the exposure step of the process of manufacturing a semiconductor device.
Since the conventional photomask 70 has the same pattern 73 in shape as the designed circuit pattern, it can be sufficiently inspected by the above-described method. However, as for a phase-shifting mask which has been developed recently, there arises a problem that it is impossible to find defects thereof by the above method because the phase-shifting mask does not necessarily coincide in shape or size with a circuit pattern which is designed with CAD and the like.
In other words, a phase-shifting mask, which generally makes use of the effect of a phase shift method for improving a transferring accuracy of a pattern in the exposure step, is provided with a subsidiary pattern which is filled up with a phase reversing element (referred to as "phase shifter" hereinafter) for reversing light phase by 180.degree. besides a main pattern, and is formed so that the edge of an exposed surface of the main pattern may be clearly defined by an interference of light passing through the main and subsidiary patterns.
The pattern of the phase-shifting mask is different from the circuit pattern previously designed in shape and size, therefore it is impossible to find any defect of the phase-shifting mask by checking both pattern images in a conventional inspecting system wherein the photomask is imaged just as it is by an ordinary white light to obtain image data to be inspected.
Further, more concrete description will be followed.
FIGS. 11(a) and 11(b) each illustrate a principal part of a phase-shifting mask.
FIG. 11(a) is a perspective view of a principal part of a phase-shifting mask 100 and FIG. 11(b) is a vertical section of the same.
A light-shielding film 102 is formed on a glass substrate 101 by evaporating a metal such as chromium, being provided with a main pattern 103 disposed substantially at the center thereof and subsidiary patterns 104 around the main pattern 103 parallel with each side of the main pattern 103 at a predetermined distance therefrom. Each of the subsidiary patterns 104 is filled up with a phase shifter 105 such as SiO.sub.2.
The phase shifter 105 is made of transparent or translucent material. When light passes through it, a phase of the passing light is shifted by 180.degree. in comparison with the case where light does not pass through it.
Each of the subsidiary patterns 104 is disposed so that the zero-order diffraction light from the main pattern 103 and the Fresnel diffraction light from the subsidiary pattern 104 may enhance each other when a ray of light having predetermined wavelength is projected, thereby clearly defining an edge of the pattern on an exposed surface and ensuring pattern transfer with high accuracy.
The image which is obtained when the phase-shifting mask 100 is imaged by the image pickup device 90 using an ordinary light source as shown in FIG. 10 can be seen like an image 106 in FIG. 12(a), and obviously differs in shape from a pattern 108 of a circuit pattern 107 shown in FIG. 12(b) which is initially designed.
Hence, it is impossible to detect any defect of the phase-shifting mask 100 by checking both pattern images.